The present invention relates generally to a computerized nuclear magnetic resonance (NMR) imaging system, and more particularly, to a function generator for generating complex analog waveforms used in an NMR imaging system and control signals necessary for its operation.
Nuclear magnetic resonance finds applications in numerous fields. The present invention will, however, be introduced by reference to NMR imaging in medicine. NMR imaging is a powerful tool which can reveal the structure and make-up of internal tissue and organs of a body through a non invasive procedure. The physician is thus provided with information which can be used to identify the nature of the disease that afflicts the internal organ and pinpoint its location.
NMR imaging, very broadly, operates as follows. A patient is subjected to the influence of carefully aligned magnetic fields produced by the NMR imaging system. The fields force nuclei within the body to orient themselves along lines of force associated with the magnetic fields. When the fields are removed, the nuclei relax and their relaxation is accompanied by a characteristic electromagnetic radiation which can reveal the molecular composition of a given tissue. The electromagnetic radiation possesses a spatial distribution and a timing and frequency relationship can be used for imaging the position, shape and composition of selected internal organs or tissue of the patient.
An NMR imaging system, very broadly, includes magnets which surrounds the patient, radio frequency (RF) coils and power drivers which are used for producing magnetic fields in the magnet, a receiver for detecting the patient emitted electromagnetic radiation and for producing electrical signals representative thereof, and a processor for analyzing the data from the receiver and for generating an image from such data.
While the NMR phenomena has been observed as early as 1946, the ability to create images from the spatial distribution of NMR signals was not demonstrated until the early 1970's.
The difficulties associated with constructing an operational NMR imaging system reflects the enormous complexity involved in sensing the rather feable NMR radiation from a patient and using these feable signals to construct a discernable image which can be interpreted by the physician. Another difficulty concerns the generation of complex waveforms for driving the RF coils and power drivers in the magnets to generate precisely controlled and aligned magnetic fields so that the NMR signals from the patients could be properly interpreted.
The present invention is specifically related to a function generator which simplifies the generation of the complex analog waveforms and which provides an increased level of flexibility. With the invention, the scientist or physician can exercise real-time control over the generation of the complex waveforms referred to above. The invention's objective is to provide the function generator of the present invention at a configuration which reduces its size and cost and increases its flexibility over prior art devices.
The principles which determine: the shape and type of magnetic fields required in an NMR imaging system; the nature, characteristics and frequencies of the electromagnetic radiation emitted by the patient; and the basic hardware necessary for sensing and analyzing the NMR signals from the patient have been described in the literature.
For example, the article entitled "Nuclear Magnetic Resonance: Beyond Physical Imaging", by Paul A. Bottomley and published in the February, 1983 issue of IEEE's Spectrum magazine, pages 32-38, describes the basic NMR principles. In FIG. 5, it shows the basic hardware blocks, including a transmitter, receiver, magnet, gradient coils and a computer for analyzing the data. The article by Ian L. Pykett entitled "NMR Imaging in Medicine", appearing in the May, 1982 issue of Scientific American, pages 78-88, similarly provides a description of the basic principles of NMR. The hardware structure of a typical NMR system is further illustrated in an article entitled "Electronics and Instrumentation for NMR Imaging", authored by A. A. Maudsley et al and published in the August, 1984 issue of IEEE transaction on nuclear science, Vol. NS-31, No. 4, pages 990-993. The overall configuration of an NMR imaging system appears in FIG. 1, and shows the basic interconnection between the transmitter, receiver, waveform controller, magnet and the computer which invariably appear in systems of this type. While this article alludes (at the right column of page 990) to the desirability of loading the shape of a waveform from a controller, the flexible, modular and highly compact structure of the waveform generator in accordance with the function generator of the present invention is not shown.
The ability to manipulate, alter and adjust the analog waveforms which drive the RF coils and the gradient power supply coils is important. These waveforms define the resolution of the obtained image and determine the size, location and angular view of the displayed image. The desirability of having maximum flexibility in generating this waveform is therefore readily apparent.
Yet, in known prior art devices, the complex waveforms are generated by dedicated circuits which are designed to deliver waveforms of predetermined characteristics. These dedicated circuits are often under the general supervision of a central controller which can determine when the waveforms are produced but cannot dynamically control the exact and final shape of these waveforms. While in known prior art devices the central controller or computer is capable of selecting a desired analog waveform from a library of such waveforms, the exact shape, timing and frequency of the waveform is not under computer control.
A notable disadvantage of NMR imaging is that the patient's exposure time to the magnetic fields and the ensuing processing period are rather long, i.e. in the order of 1 to 10 minutes. Further, when an image is obtained, it may be evident that the image does not show a desired portion or view of a patient's anatomy. It is thereafter necessary to change the position of the patient within the magnet and/or select a different configuration of RF fields from a fixed set of fields which, in prior art devices, are permanently wired in the dedicated circuits.
Thus, prior art NMR devices are inflexible. They restrict the options available to the diagnostician and cannot be easily refurbished to accommodate advances in the art related to the generation of improved or better defined magnetic fields.
Further, hard-wired, fixed, dedicated circuits present the additional disadvantage that hardware repair or modification is difficult and retrofitting to accommodate advances in the understanding of the NMR phenomena are almost impossible.
The primary objective of the present invention is therefore to provide a function generator for an NMR imaging system which avoids the disadvantages of similar circuits found in prior art devices.